The invention relates to a method and a device for suppressing disturbances in a signal transmitted on a transmission line, in particular for suppressing disturbances in a PCM (Pulse Code Modulation) signal.
PCM transmission lines are increasingly used for transmitting digital data over long distances. Increasing the distance over which data are transmitted makes it more difficult to meet the requirements regarding the suppression of interference, noise, jitter, and other disturbances on the transmission line. Long transmission lines may also result in an increased amplitude damping for the electronic components connected to the transmission line.
A reduced supply voltage results in a severe reduction of the available voltage modulation range. If, for example, the worst case supply voltage of a device is 4.75 V and the voltage modulation range is reduced by 1 V with respect to the supply potential and the ground potential, respectively, the resulting voltage modulation range is 2.75 V. If, however, the worst case supply voltage is 3.15 V and the voltage modulation range is reduced by 0.8 V with respect to the supply potential and the ground potential, respectively, the resulting voltage modulation range is only 1.55 V. The available voltage modulation range of an input signal at an input of an electronic component may furthermore be reduced by ESD (Electrostatic Discharge) protection diodes. The ESD protection diodes result in a reduced range for the signal voltage amplitudes at the input of the electronic component. This leads to a low signal-to-noise ratio which even declines when long transmission lines are used, since the noise environment is kept to be the same due to required noise resistances in applications. In addition, the signal-to-noise ratio deteriorates as the length of the transmission line increases.
The noise at an input of an electronic component includes the self-noise at the input of the electronic component and the interference caused by disturbances picked up by transformers used for coupling signals into the transmission line. The disturbances picked up by the transformer are a major part of the NEXT (near-end cross talk) noise. The problem of cross talk is for example discussed in the article xe2x80x9cStatistical Behavior of Crosstalk Power Sum with Dominant Componentsxe2x80x9d in Bell Syst. Tech J., Vol. 60, No. 7.
A further problem arises from the temperature requirements that a transmission system must meet. The electronic components and the transmission line must operate in a temperature range of, for example, xe2x88x9240xc2x0 C. to +85xc2x0 C. without violating a maximum allowable bit error rate that the transmission system must be able to tolerate.
A transmission system furthermore has to be able to meet its transmission requirements even when a sending unit varies the amplitude of the sending signal. The PCM24 standard, for example, allows a variation of the amplitude of the sending signal in the range of between 2.4 V and 3.6 V in a time period of 102xc3x97650 nsec.
A prior art receiver receiving a signal is, for example, configured such that the signal amplitude is adjusted to have a desired value. A group delay equalization is performed in accordance with a fixed group delay equalization function that is based on known transmission properties of the transmission line. The transmission line is for example a cable whose transmission properties have been measured. In other words, if the correct amplification, which results in the desired amplitude, has been found, then the group delay equalization is also adjusted correctly. This adjustment method however presupposes that specific types of transmission lines or cables with known properties, predetermined amplitudes at the transmitter, and specific external circuits are used.
After the adjustment of the amplitude and the group delay of the received signal, the thus reconstructed signal fluctuates within boundaries due to noise, due to an amplitude modulation of the input signal, due to temperature changes, or due to any other disturbances. If the reconstructed signal fluctuates beyond these boundaries, then the signal may be adjusted again. German Published, Non-Prosecuted Patent Application DE 197 35 752 A1 describes such a system that readjusts the signals as soon as they leave their boundaries. A problem of a system that works as described above is that the modulation range may be severely restricted, as is described in the article xe2x80x9cA 284 kbit/s ISDN Burst Transceiverxe2x80x9d, in IEEE Journal of Solid State Circuits, Vol. SC 22 No. 6, 1988.
It is accordingly an object of the invention to provide a method and a device for disturbance suppression in a signal which overcome the above mentioned disadvantages of the heretofore-known methods and devices of this general type and which effectively suppress modulation disturbances and at the same time keep bit error rates low.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for suppressing disturbances in signals transmitted on a transmission line, which comprises receiving signals having an amplitude; adjusting the amplitude of the signals with coarse adjustment steps and with fine adjustment steps for a disturbance suppression; subsequently comparing the amplitude adjusted signals to at least a lower reference level and an upper reference level for determining a first percentage of the amplitude adjusted signals violating the lower reference level and a second percentage of the amplitude adjusted signals violating the upper reference level; selecting the coarse adjustment steps and the fine adjustment steps in accordance with an adjustment characteristic which evaluates at least the first percentage and the second percentage; changing the adjustment characteristic if a tendency of reference level violation of the amplitude adjusted signals is detected; and checking it the step of changing the adjustment characteristic improves the disturbance suppression.
In accordance with another mode of the invention, the adjustment characteristic is changed such that an adjustment range of the fine adjustment steps is increased.
In accordance with a further mode of the invention, modulation disturbances are suppressed with the step of adjusting the amplitude of the signals.
In accordance with yet another mode of the invention, the coarse adjustment steps are per formed with a filter selectively operating with a first filter stage suited for signals received from a transmission line having a first length and opera ting with a second filter stage suited for signals received from a transmission line having a second length, the second length being greater than the first length; the fine adjustment steps covering a first amplitude range, when the filter operates with the first filter stage and covering a second amplitude range when the filter operates with the second filter stage, the first amplitude range being greater than the second amplitude range.
In accordance with a further mode of the invention, the first amplitude range is extended by a range of at least 3 dB beyond a lower boundary and beyond an upper boundary of the second amplitude range.
In accordance a yet a further mode of the invention, the range of the at least 3 dB is divided into three fine adjustment steps each having a step width of at least 1 dB.
In accordance a yet another mode of the invention, the adjustment characteristic is changed in dependence of the step of checking if the disturbance suppression has improved.
With the objects of the invention in view there is also provided a device for suppressing disturbances in signals transmitted on a transmission line, comprising an equalizer receiving signals having an amplitude and adjusting the amplitude of the signals; the equalizer having a gain control device for providing fine adjustment steps and having a filter connected to the gain control device, the filter providing coarse adjustment steps; a level detector connected to the equalizer for comparing the amplitude adjusted signals to at least a lower reference level and an upper reference level; a control device connected to the equalizer for determining a first percentage of the amplitude adjusted signals violating the lower reference level and a second percentage of the amplitude adjusted signals violating the upper reference level, for detecting a tendency of reference level violation of the amplitude adjusted signals, for controlling the equalizer to perform the coarse adjustment steps and the fine adjustment steps in accordance with an adjustment characteristic which evaluates at least the first percentage and the second percentage, and for changing the adjustment characteristic on detection of the tendency of reference level violation of the amplitude adjusted signals.
In accordance with another feature of the invention, the control device changes the adjustment characteristic by increasing an adjustment range of the fine adjustment steps. In accordance with yet another feature of the invention, the filter has a first filter stage suited for signals received from a transmission line having a first length and has a second filter stage for signals received from a transmission line having a second length, the second length being greater than the first length, the fine adjustment steps covering a first amplitude range, when the filter operates with the first filter stage and covering a second amplitude range when the filter operates with the second filter stage, the first amplitude range being greater than the second amplitude range.
Although the invention is illustrated and described herein as embodied in a method and a device for disturbance suppression, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.